In recent years, steering-by-wire steering systems of this kind have been actively developed. Japanese Patent Application Publication No. 7-47970, for example, describes a controller for a vehicle steering system that estimates the driving skill of a driver under certain driving conditions at various time points, and changes control characteristics of the steering system in accordance with the estimated driving skill to control the steering system in accordance with driving conditions of the vehicle, thereby always optimally controlling a steering system of a vehicle that is significantly affected by driving skills. This controller for a vehicle steering system can always optimally estimate the driving skill of a driver regardless of the driving conditions of the vehicle, by determining the actual trajectory of the vehicle from turning behavior of the vehicle, determining a desired trajectory from the operation performed by the driver, or from a driving path of the vehicle on a map, and estimating the driving skill of the driver by comparing the integral of the deviation of the actual trajectory from the desired trajectory with a predetermined value every predetermined period of time.
In general, when a driver who has driven a vehicle equipped with a steering system (hereinafter referred to as the conventional steering system) that steers the steered wheels by a certain steering amount mechanically determined relative to the amount of operation of a steering wheel switches to a vehicle equipped with a steering system (hereinafter referred to as the actual steering amount-variable steering system) that can change the amount of actual steering of the steered wheels relative to the amount of operation of the steering wheel, the driver is highly likely to feel a sense of discomfort with the difference between the steering characteristics of these steering systems. Specifically, with the actual steering amount-variable steering system, it is possible to steer the steered wheels by a large steering amount even when the amount of operation of the steering wheel by a driver is small. Thus, for a driver who knows the steering characteristics of the actual steering amount-variable steering system well, that is, who is familiar with the steering characteristics thereof, excellent operability is obtained because the amount of operation of the steering wheel can be reduced.
However, a driver who does not know the steering characteristics of the actual steering amount-variable steering system well, that is, who is not familiar with the steering characteristics thereof can feel a sense of discomfort with such actual steering action of the steered wheels. Specifically, even a driver with excellent driving skills can feel a sense of discomfort with the steering characteristics, and feel it difficult to drive the vehicle until the driver becomes familiar with the steering characteristics (actual steering action) typical of the actual steering amount-variable steering system after a switch from the steering characteristics of the conventional steering system. Accordingly, it is important for a driver to become familiar with the steering characteristics typical of the actual steering amount-variable steering system in order to eliminate the sense of discomfort, and to feel it easy to drive.
However, the period (time) required for a driver to become familiar with the steering characteristics typical of the actual steering amount-variable steering system, and the degree of familiarity with the typical steering characteristics significantly vary depending on the difference in the driving background of the drivers, such as the experience in driving a vehicle. Specifically, a driver who has excellent driving skills and is familiar with driving of vehicles is highly likely to soon become familiar with the steering characteristics typical of the actual steering amount-variable steering system. On the other hand, a driver who has less opportunity to drive vehicles is highly likely to take a long time to become familiar with the steering characteristics typical of the actual steering amount-variable steering system. Accordingly, with regard to the actual steering amount-variable steering system, in consideration of the fact that the familiarity degree differs depending on the driver driving the vehicle, it is desired that excellent operability be obtained for all drivers, that is, that driving of vehicles become easy.